The determination of the chlorine content in water, especially in the water of swimming pools is an important and widely used control reaction. The most current method used for this purpose is the reaction between chlorine and ortho-tolidine (OTO), a yellow color being produced by oxidation of the molecule by chlorine. The intensity of the resulting coloration in a given volume of the tested water is compared with an empirically established color scale, thus giving a quantitative estimate of the chlorine content. The sensitivity of the reaction is high, but the pale yellow shade produced by concentrations under 0.5 parts per million of chlorine renders an exact estimation a little difficult.
The solutions of OTO are sufficiently stable so that this form of the reagent, i.e. solution, can be used for an easy testing in small private swimming pools.
However, an objection has been raised against the use of OTO for this purpose. Its cancerogenic activity has been experimentally established and this fact has cast a shadow on its wide-spread use. Attempts to replace OTO, which gives a highly sensitive and specific reaction with chlorine, by another substance have not been very successful, so far. The only substance, discovered by Palin, which has found some use for satisfactory chlorine determinations is assym.diethyl-paraphenylene-diamine (DPD). A certain drawback is its strongly allergenic character which necessitates a certain caution in its manipulation. The commercial form is DPD tablets sealed in a metallic foil. They are used in connection with the Lovibond comparator. This makes their use considerably more expensive than the use of OTO in the current cheap test kits.
It has now been found that, under certain conditions, which are the object of the present patent application, another member of the phenylene-diamine group can be advantageously used for routine chlorine testing. It does not possess the toxicity of DPD and its allergenicity is, under the conditions of the reaction, insignificant. The chemical composition is para-ethyl-oxyethylamino-aniline (EOAA), and it is used preferably in the form of its salts. It is generally more soluble than DPD, very easily oxidizable and its molecule is much more rapidly destroyed than that of DPD. These properties preclude the use of the base and its salts in solutions similar to those of OTO. The stability of EOAA salts in various organic solvents (e.g., alcohols, acetone, glycols) or acidic solutions in water (even in the presence of complexing agents) is of the order of several minutes to about one month in a closed container. A repeated exposure of the solutions to air by opening the containers for sampling, cuts down the stability still more, so that a stable EOAA reagent cannot be prepared in this way. The only exception is the solution of EOAA in glycerine (BP quality) which keeps stable over one year and lends itself to possible practical use.
It has also now been found that the salts of paraethyl-oxyethylamino-aniline (EOAA) can be brought, as solids, into a form which is most practical and useful for chlorine determinations and which is, under appropriate preserving conditions quite stable. The form of tablets has not been found very practical because of the relatively slow disintegration of the tablets and the necessity of mixing the tested sample.